Commercial ion selective electrodes (ISE) are a class of chemical sensors that are presently expensive, fragile, bulky, and require periodic maintenance. There is considerable ISE research being done to miniaturize them, and in so doing, make them more rugged, less expensive, and in some cases even disposable. There are many technology areas that can greatly benefit from these improved ISE devices such as the medical field, environmental chemistry, analytical chemistry, process analytical chemistry, and the chemical manufacturing industry. With the development of microsensors, arrays of multiple sensors can be made quite easily and cheaply. These arrays can be incorporated into on-line process analyzers, giving them increased reliability, accuracy, capability, and cost effectiveness. Maintenance costs would be minimized if they can be made disposable. In the medical and clinical fields, the chemistry of individual cells can be studied and the ionic composition of a patient's blood can be monitored on-line during surgery.
Progress in the development of stable micro ISE devices has been hampered by several fundamental problems. Commercial electrodes have a built-in stabilizing internal reference system based on either Ag/AgCl reference elements immersed in aqueous reference solutions or an inert platinum (or gold) wire immersed in an aqueous solution containing a redox couple. Without this internal reference system, the potential of the electrode drifts, decreasing the accuracy of the measurement and requiring frequent calibration.
Most commercial liquid membrane ISE devices have an ion sensitive membrane in direct contact with the internal reference solution. In making this electrode, the membrane is formed first, sealed to the electrode body, then it is filled with the internal reference solution. This method is not easily miniaturized.
The simplest way to miniaturize an ISE device is to eliminate the internal reference system and put the membrane in direct electrical contact with the conductor. Microelectrodes of the coated wire type (CWE), obtained by coating a wire with an ion sensitive membrane, are commonly reported in the literature as being quite functional, but suffer potential drift on the order of mV/hr; see "Coated-Wire Ion-Selective Electrodes" in Ion-Selective Rev. 1984, Vol. 6, pp 125-172; Pergamon Press Ltd. Electrodes having an internal reference system show drifts of uV/hr or less.
Chemical microsensors called ISFET's (Ion Sensitive Field Effect Transistor) are made by directly attaching an ion sensitive membrane to the gate of an FET. Besides being very small (about 2mm.sup.2) the microsensor chip is inexpensive, rugged, and is of low output impedance. Again, due to the lack of an internal reference system, these devices are subject to potential drifts.
It is apparent that one cannot simply scale down a conventional ISE since, at small dimensions, it is difficult to attach a membrane due to the small volume. Water evaporation through the membrane is significant. Complete loss of the internal reference solution occurs in a few days, depending on membrane thickness, rendering the device inoperable.
There are numerous papers and patents describing attempts to build an internal reference system into micro devices. For CWE, a polyvinyl alcohol (PVA) gel containing KCl was used, over which the membrane was cast; see "Miniature Solid State Potassium Electrode for Serum Analysis" in Analytical Chemistry, Vol. 45, No. 9, Aug. 1973. It was noted in this paper that special care was needed in order to prevent the membrane from bursting. The patent literature presents disclosures of small electrodes having a "dried" internal reference system which becomes active when moisture diffuses into it through the outer membrane: U.S. Pat. Nos. 3,856,649; 4,214,968; 4,340,457; 4,487,679; 4,571,293; 4,578,173; and 4,615,788. At best these systems seem marginally effective.